No unique broadcasting and/or operating standard is currently available as relates to composite synchronism video signals.
As a result, for proper operation of video sets, the latter must be equipped with devices which be capable of detecting in an automatic manner the type of the input signal, or more specifically, the repeat frequency fh of the video picture line, which is representative of the broadcasting standard.
For instance with monitors, the composite synchronism video signal contains n impulses at the horizontal line frequency fh, followed by a single impulse at the vertical frequency fv, the duration whereof is equal to a multiple n' of the duration of the line frequency impulses and tied to the broadcasting standard being used.
The prior art has indeed proposed a solution to the problem of finding the line frequency fh.
This prior solution consists of providing a counter, having a predetermined scan frequency fc higher than the line frequency fh, and of detecting the number of count impulses occurring between two successive impulses at line frequency.
By comparison of two chronologically successive measurements it becomes possible to decide whether a frequency change did occur.
However, this prior solution cannot prevent the measurement from being affected by the presence of vertical synchronization impulses.